IOWA STATE COLLEGE OF 
AGRICULTURE AND MECHANIC ARTS 



SOIL ACIDITY AND BACTERIAL ACTIVITY 

A DISSERTATION SUBMITTED TO THE GRADUATE FACULTY 

IN CANDIDACY FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY 



BY 
R. E. STEPHENSON 



NO. 7 



Reprinted from Soil Science 
Vol. XII, No. 2, August. 1921 






<5 



Reprinted from Soil Science 
Vol. XII, No. 2, August, 1921 



SOIL ACIDITY AND BACTERIAL ACTIVITY 1 

R. E. STEPHENSON 
Agricultural College, University of Kentucky 

Received for publication February 17, 1921 * 
INTRODUCTION 

Just how and why soils become acid is a problem that has not yet been 
definitely solved. Neither is the effect of reaction upon the activity of soil 
organisms clearly understood. But is has been fairly well established that the 
process of nitrification once thought to be absent in acid soils, does proceed 
to an appreciable extent. In fact nitrification is perhaps sufficient for normal 
crop production, in most cases, provided the supply of organic matter is ade- 
quate. The process of ammonification which of course must precede nitri- 
fication is carried on by so many classes of organisms that it is not usually a 
limiting factor in crop production in either acid or sweet soils, under aerobic 
or anaerobic conditions. 

In practically all soils there must be two analytical processes, the decompo- 
sition of organic matter, and the disintegration of minerals. The importance 
of microorganisms in bringing about these processes is too obvious to need 
comment. While these processes are occurring, plant growth also takes place. 
The general tendency of plant growth has been found to be to keep the nutri- 
ent solution nearly neutral. Crop production therefore doubtless has a tend- 
ency to prevent soils from becoming acid in reaction, while the leaching of 
bases has the opposite effect. 

The cultivation of soils for crop production of course encourages leaching, 
stimulates bacterial activity, and on the whole in this indirect way must tend 
to produce acid soils. 

In mineral disintegration, with the accompanying interchange of ions, both 
acids and bases must be set free. Similar effects are produced when organic 
matter is broken down. But changes in the organic portion of the soils 
must occur under favorable conditions much more rapidly than changes in 
the mineral portion. The acids and carbon dioxide produced in organic decay 
hasten mineral disintegration, and therefore increase the availability of mineral 

1 Part of the results of this study on "Soil Acidity and Bacterial Activity" has already 
been published. Two papers, "The Effect of Organic Matter on Soil Reactions. I," 
and "The Activity of Soil Acids" were published in Soil Science (6, 7), another paper "Nitri- 
fication in Acid Soils" is in press at the Iowa Agricultural Experiment Station. This paper 
is the first part of a thesis presented to the graduate faculty of the Iowa State College of 
Agriculture in partial fulfilment of the requirements for the degree of Doctor of Philosophy. 

133 

BOIL SCIENCE, VOL. XII, NO. 2 



134 R. E. STEPHENSON 

plant-food. But though minerals are put into solution by these processes, 
there is also a compensating effect, in that organic decomposition products 
are capable of forming insoluble compounds with the minerals disintegrated 
and thus may prevent or at least retard the loss of the minerals by leaching. 

One fact to be kept in mind in connection with both organic acids and with 
bases, is that so far as available data indicate, these compounds do not remain 
long as such. Oxidation changes convert the nitrogen bases to nitric acid 
and the organic acids to carbon dioxide. Only the nitric acid produced, there- 
fore, is capable of causing a permanent direct effect upon soil reaction. Min- 
eral bases and acids, on the other hand, are permanently removed from the 
soil primarily by leaching. The portion used by the plant may be expected 
to be returned to the soil, at least in part. 

It may be observed, too, that practical experience demonstrates that soils 
containing sufficient organic matter remain more productive for a longer time 
than those soils which are low in organic matter. Loss of organic matter is 
likely to result in a sour, soggy, infertile soil, which does not respond to tillage 
or commercial fertilizer. Muck and peat soils are notable exceptions but 
largely because mineral elements, such as potassium and other bases, were 
never present. And again, such soils occur only under those conditions which 
favor a large production of organic acids, and prevent complete oxidation. 
These soils, therefore, are often highly acid, and this condition is undoubtedly 
due mainly to organic acids. But by way of contrast it must be observed 
that sandy soils and heavy clays, which do not contain sufficient organic 
matter to produce an appreciable acidity, are often highly acid and non- 
productive. 

In this work one heavy silt loam soil, one soil somewhat sandy, both low in 
organic matter, and a loam soil rather high in organic matter were used, for 
the purpose of studying the changes which occur, the rate of change, and to 
some extent the final products of the reactions. 

HISTORICAL 

Previous investigations of the effect of organic matter upon the reaction of 
soils is very limited in amount and application. White (8), Skinner and 
Beatty (3), Miller (2) and Stephenson (6) found no positive evidence that the 
decay of organic matter in ordinary soils under conditions which would be 
favorable to crop production, produced any appreciable increase in the lime 
requirement of the soil. No argument is necessary, of course, to establish 
the fact that the large production of nitric acid would increase the acid reac- 
tion of the soil or use up bases rapidly if they were present. 

THE PLAN OF THE EXPERIMENT 

In a previous publication (6) the effect of the decomposition of albumin, 
casein, starch, blood, dextrose, alfalfa, and ammonium sulfate on the reaction 
of two soils was studied. Further work along this same line is reported here, 



SOIL ACIDITY AND BACTERIAL ACTIVITY 135 

with organic materials of more general use such as farm manure, cottonseed 
meal, horse manure, timothy hay, clover hay, green timothy and green clover. 
Opportunity is thus afforded for comparing the green and the more matured 
dried materials. 

Two of the same soils used in the earlier work were employed, one rather 
sandy and light in color, the other dark and fairly rich in organic matter, and 
of the loam type. Applications of the various materials were made at the 
rate of 10 tons per acre of air-dried material, on the basis of 2,000,000 pounds 
of soil per acre. The coarse materials were ground and thoroughly mixed with 
the soils, in 1 -gallon earthenware jars. Samplings were made at intervals of 
2, 5, 10, 15, and 22 weeks, respectively. Two series were run, one limed and 
the other unlimed. Determinations were made at each sampling for the 
ammonia, nitrates, acidity, and residual carbonates, since these are directly 
connected with the effect of materials on the soil reaction. A test was made 
at the second sampling, for the soluble non-protein nitrogen present in one 
of the soil types. This test should throw some light on the question of the 
possibility of any accumulation of soluble products of protein decomposition, 
other than nitrates and ammonia, and should also show whether there is any 
correlation between these products and the quantity of nitrates or ammonia 
present in soils. 

AMMONIFICATION 

The quantity of ammonia was determined by the aeration method, potassium 
carbonate being used to liberate the ammonia. Incidentally it may be said 
that experience at the Iowa Agricultural Experiment Station with this method 
would lead to the conclusion that those workers who have found the method 
unsatisfactory, must have experienced a faulty manipulation. The secret of 
successful operation of the method, is that the aeration must stir the soil 
completely to the bottom of the containing flask. The results of the ammonia 
determinations are given in table 1. 

It may be observed that there is very little accumulation of ammonia with 
any of the treatments except the cottonseed meal. It has shown the greatest 
accumulation of ammonia at the first sampling and a greater accumulation 
when the soil was untreated, than when it was limed, both of which results 
agree with work done previously (6) with highly nitrogenous materials. 
There is too small an accumulation of ammonia on the untreated soils to show 
marked differences between the limed and the unlimed soils. The same may 
be said of most of the other treatments, though there is a greater amount 
of ammonia in the unlimed soils where green manures were added. The 
greatest amount of ammonia is found in nearly all cases at the first sampling 
before nitrification is well started. There is quite a marked difference in the 
two soils, noticeable where the cottonseed meal is used, in that the amount 
of ammonia throughout the test remains high on the unlimed sandy soil, 
while on the humus soil nitrification seems to have just about kept pace with 



136 



R. E. STEPHENSON 



ammonification even in the absence of lime. This result lends support to 
the belief that soils containing sufficient organic matter are more active bac- 
teriologically, and likewise usually more productive, than soils containing 
less organic matter even when the total time requirement is much greater 
for the organic soils. 

The amount of ammonia produced may depend upon several factors. But 
when conditions are favorable for nitrification the ammonia is changed to 
nitrates almost as rapidly as produced. 

TABLE 1 
Amount of ammonia at the end of each period 





FIRST SAMPLE, 
2 WEEKS 


SECOND SAM- 
PLE, 5 WEEKS 


THIRD SAMPLE, 
10 WEEKS 


FOURTH SAM- 
PLE, 15 WEEKS 


FIFTH SAMPLE, 
22 WEEKS 


AVERAGES 




No 
lime 


Lime 


No 
lime 


Lime 


No 
lime 


Lime 


No 
lime 


Lime 


No 

lime 


Lime 


No 
lime 


Lime 


Humus soil : 

Soil alone 

Cottonseed 

meal 

Manure 

Timothy hay 
Clover hay . . . 
Green timothy. 
Green clover. . 


p. p.m. 
11.8 

302.4 

8.4 

5.6 

19.6 

44.8 

33.6 


p. p.m. 

14.0 

285.6 

5.6 

8.4 

11.2 

11.2 

14.0 


p. p.m. 
11.2 

268.8 
11.2 
11.2 
8.4 
14.0 
16.8 


p. p.m. 

8.4 

61.6 
8.4 

11.2 
5.6 
5.6 
5.6 


p. p.m. 
16.8 

98.0 
11.2 
16.8 
11.2 
16.8 
16.8 


p. p.m. 
11.2 

22.4 
11.2 
11.2 
11.2 
11.2 
11.2 


p. p.m. 
11.2 

86.8 
14.0 
11.2 
5.6 
11.2 
11.2 


p. p.m. 
14.0 

19.6 

11.2 

5.6 

11.2 

11.2 

8.4 


p. p.m. 
11.2 

32.0 
14.0 
11.2 
11.2 
11.2 
11.2 


p. p.m. 
8.4 

14.0 
11.2 
11.2 
11.2 
11.2 
8.4 


p. p.m. 
13.4 

157.6 
11.7 
11.2 
11.2 
19.6 
17.9 


p. P.m. 
11.1 

80.6 
9.6 

9.5 
10.1 
10.1 

9.5 


Average. . . . 


61.6 


50.0 


48.8 


15.2 


22.8 


12.8 


21.6 


11.6 


14.6 


10.8 


48.5 


28.1 



Sandy soil: 


























Soil alone 


56.0 


30.8 


14.0 


5.6 


16.8 


11.2 


19.6 


11.2 


14.0 


14.0 


24.1 


14.6 


Cottonseed 


























meal 


294.8 


305.2 


280.0 


100.8 


132.5 


16.8 


151.2 


22.4 


14.0 


19.6 


194.5 


92.9 


Manure 


16.8 


19.6 


8.4 


11.2 


8.4 


11.2 


8.4 


8.4 


11.2 


11.2 


10.6 


12.3 


Timothy hay. . 


11.2 


8.4 


11.2 


8.4 


16.8 


11.2 


14.0 


11.2 


11.2 


89.6 


12.9 


25.8 


Clover hay . . . 


39.2 


39.2 


19.6 


11.2 


14.0 


14.0 


11.2 


8.4 


16.8 


14.0 


20.1 


17.4 


Green timothy. 


58.8 


47.6 


33.6 


16.8 


14.0 


8.4 


11.2 


8.4 


5.6 


5.6 


24.6 


17.8 


Green clover. . 


103.6 


75.6 
75.2 


39.8 


14.0 


11.2 


11.2 
12.0 


11.2 


11.2 


5.6 


5.6 


32.5 


23.5 


Average. . . . 


97.2 


56.8 


24.0 


30.5 


32.4 


11.6 


17.2 


22.8 


45.6 


29.2 



Lime favors nitrification and at least in that indirect way indicates a 
retarded ammonification. Lime also increases the number of organisms, and 
should therefore tend to reduce the total of ammonia and nitrates in the pres- 
ence of a limited supply of organic matter, because of greater nutritional 
demands by the increased number of organisms. When a large amount of 
nitrogenous organic matter is added perhaps this would not result. And since 
the ammonification process is the actual limiting factor under conditions which 
permit of nitrification, the increased basicity due to the use of lime evidently 
does have a retarding effect. 



SOIL ACIDITY AND BACTERIAL ACTIVITY 



137 



When averages are taken of all determinations and all treatments, there is 
no case on the humus soil (so-called because of its higher content of organic 
matter) where lime has not diminished the amount of ammonia produced. 
On the sandy soil there are two cases, with manure and with timothy hay, 
where the reverse is true, but the result would appear to be more nearly acci- 
dental than fundamental. 

TABLE 2 
Nitrates at each successive sampling 



TREATMENT 


FIRST SAMPLE, 
2 WEEKS 


SECOND SAM- 
PLE, 5 WEEKS 


THIRD SAMPLE, 
10 WEEKS 


FOURTH SAM- 
PLE, 15 WEEKS 


FIFTH SAMPLE, 
22 WEEKS 


AVERAGE 




No 

lime 


Lime 


No 

lime 


Lime 


No 

lime 


Lime 


No 
lime 


Lime 


No 
lime 


Lime 


No 

lime 


Lime 


Humus soil 

Soil alone 

Cottonseed 

meal 

Manure 

Timothy hay. ' 
Clover hay . . . 
Green timothy. 
Green clover. . 


P- p.m. 
28.6 

33.0 
14.2 
Tr.* 
40.6 
45.6 
69.4 


p. p.m. 
19.1 

45.7 
7.3 
Tr. 
58.9 
51.5 
78.1 


p. p. m. 
63.5 

98.3 

21.4 

Tr. 

67.8 
100.4 
109.7 


p. p.m. 
68.8 

243.2 
23.8 
Tr. 
92.5 
83.9 

122.0 


p. p.m. 

38.9 

214.8 

37.8 

Tr. 

80.3 

180.5 

234.1 


p. p.m. 
95.9 

309.0 
57.8 
20.5 
129.5 
125.0 
319.1 


p. p.m. 

52.3 

302.4 

36.7 

Tr. 

86.3 

141.1 

181.5 


p. p.m. 
102.0 

289.9 
61.8 
35.5 

133.5 
93.8 

168.1 


p. p.m. 

50.0 

324.0 

74.5 

22.8 

116.7 

181.4 

284.6 


p. p. m. 
121.1 

316.0 
104.1 
67.4 
170.8 
121.4 
201.0 


p. p.m. 
64.7 

194.5 
36.9 

4.5 

78.3 

129.8 

175.8 


p. p.m. 
83.4 

240.8 
50.9 
24.7 

117.0 
94.9 

177.6 


Average. . . . 


33.1 


37.2 


65.9 


90.6 


112.3 


150.9 


114.3 


127.8 


150.6 


165.9 


95.2 


112.8 



Sandy soil: 


























Soil alone 


17.7 


16.6 


58.6 


72.4 


85.0 


58.8 


97.6 


73.1 


81.6 


103.8 


68.1 


65.9 


Cottonseed 


























meal 


9.4 


7.3 


112.2 


138.3 


167.9 


229.4 


267.6 


400.2 


312.4 


457.4 


173.9 


246.5 


Manure 


11.2 


19.1 


38.2 


52.1 


53.1 


62.1 


61.4 


68.8 


61.8 


89.8 


45.0 


54.4 


Timothy hay. . 


Tr. 


Tr. 


Tr. 


Tr. 


Tr. 


14.8 


Tr. 


41.5 


21.3 


50.3 


4.2 


21.3 


Clover hay . . . 


11.5 


15.1 


63.6 


97.1 


83.5 


69.5 


90.7 


122.0 


123.1 


152.4 


74.5 


91.2 


Green timothy. 


12.1 


23.3 


66.1 


86.6 


100.7 


82.4 


92.0 


88.0 


105.3 


144.3 


75.2 


84.9 


Green clover. . 


16.4 


13.7 


86.0 


109.3 


153.3 


117.9 


147.3 


135.5 


207.4 


183.7 


122.1 


112.0 


Average. . . . 


11.2 


13.6 


60.7 


79.4 


90.0 


90.7 


108.0 


129.9 


130.3 


168.8 


80.4 


96.6 



; Tr. = trace. 



NITRIFICATION 



For the determination of nitrates the phenoldisulfonic acid method as modi- 
fied by Davis (1) was used. Calcium carbonate was employed to flocculate 
the soil and secure a clear nitrate. The results are given in table 2. 

It is observed that the amount of nitrates increased in the untreated soils 
up to the last sampling. 

The cottonseed meal, in accordance with its higher nitrogen content, gave a 
greater accumulation of nitrates on both soils than any other treatment. Here 
again, the sandy soil, though starting more slowly, finally ran higher than 
the better soil. On both soils, the greatest amount of nitrate was found at 



138 R. E. STEPHENSON 

the last sampling, the first two samples on the sandy soil showing less than 
the untreated soil. In most cases lime increased the nitrification of cotton- 
seed meal. 

The addition of stable manure caused a decrease in the amount of nitrates 
present in most cases, probably because of an increased number of organisms 
greater than the accompanying addition of easily nitrifiable material. 

Timothy hay had the same effect as stable manure but to a much more 
marked degree. Little nitrifiable material was added in the timothy, but 
considerable energy material was provided, and the organisms used most of 
the nitrates for nutritional purposes. The nitrates began to show at about 
the same time on both soils but never ran nearly so high as on the untreated 
soils. Lime again stimulated nitrification. The green timothy in contrast 
to the dry, stimulated nitrification at once on both soils, and the greatest 
accumulation of nitrates was found at the last sampling and in the presence 
of lime. 

Dry clover also caused a gradual stimulation of nitrification, the greatest 
effect being produced at the last sampling. The stimulation was usually 
greater also in the presence of lime. The green clover had a somewhat greater 
effect than did the dry, and maximum nitrification was induced sooner. 

When averages of all samplings and all treatments are taken, the humus 
soil shows greater nitrification in the presence of lime in every case except 
one, and this is where green timothy was applied. There is very little differ- 
ence with the green clover. When the sandy soil is considered the soil alone 
produces slightly less nitrates on the limed series. Every treatment except 
one, and in this case it is green clover, has shown greater nitrification in the 
presence of lime. Apparently the lime does not affect the nitrification of the 
green material as much as some of the dried materials. As is quite logical, 
the greatest amount of nitrates is found at the last sampling, while the great- 
est amount of ammonia is usually found at the first sampling. 

A summary of the nitrate and ammonia determinations is given in table 3. 

The table shows the largest combined production of nitrates and ammonia 
where cottonseed meal was applied, followed in order by green clover, green 
timothy, horse manure, and dry timothy, the latter two producing consider- 
ably less than the soil alone. The general effect of the lime was to decrease 
the total of nitrates and ammonia found, especially where there was any 
large production. 

ACIDITY RESULTS 

The lime requirements on the soils differently treated are given in table 
4. The determinations were made according to the modified Tacke method 
previously described (5). The acid soil was brought into contact with pure 
calcium carbonate, and the aeration and shaking continued for 10 hours before 
titrations were made. The double-titration was performed, with methyl- 
orange and phenolphthalein as indicators. 



SOIL ACIDITY AND BACTERIAL ACTIVITY 



139 



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140 



R. E. STEPHENSON 



There is little to be said in regard to the effect of the various treatments 
upon the lime requirement of the soils. The general tendency has been to 

TABLE 4 
Lime requirement of the variously treated soils in tons per 2,000,000 pounds soil 



TREATMENT 



Humus soil: 

Soil alone 

Cottonseed meal 

Manure 

Mature timothy. 

Mature clover. . . 

Green timothy. . 

Green clover. . . . 
Sandy soil: 

Soil alone 

Cottonseed meal 

Manure 

Mature timothy. 

Mature clover. . . 

Green timothy. . 

Green clover. . . . 






3.90 
3.65 
3.80 
4.05 
3.35 
4.10 
3.6 

3.20 
1.70 
2.20 
2.20 
2.15 
2.55 
1.70 



9 « 



4.20 
3.65 
4.25 
4.15 
4.15 
4.45 
4.00 

2.60 
2.15 
2.35 
2.30 
2.30 
2.65 
2.65 



3.85 
4.45 
3.60 
3.55 
3.65 
3.70 
3.25 

2.35 
2.15 
2.10 
1.80 
1.90 
2.25 
1.90 



a 



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O * 



3.80 
4.25 
3.40 
3.35 
3.25 
3.65 
3.20 

2.40 
2.50 
2.65 
2.05 
1.80 
2.30 
1.90 



3.80 
4.55 
3.80 
3.55 
3.95 
3.95 
3.85 

2.35 
2.45 
1.75 
1.75 
1.75 
2.00 
1.85 



MORE OR LESS THAN THE SOIL ALONE 



First 
sample 



-0.25 
-0.10 
+0.15 
-0.05 
+0.20 
-0.20 



-0.50 
0.00 
0.00 
-0.05 
+0.35 
-0.50 



Second 
sample 



-0.55 
+0.05 
+0.05 
-0.05 
+0.25 
-0.20 



-0.45 
-0.25 
-0.30 
-0.30 
+0.05 
+0.05 



Third 
sample 



tons 



+0.60 
-0.25 
-0.30 
-0.20 
-0.15 
-0.60 



■0.20 
■0.25 
-0.55 
■0.45 
■0.10 
■0.45 



Fourth 
sample 



+0.45 
-0.40 
-0.45 
-0.55 
-0.15 
-0.60 



+0.10 
+0.25 
-0.35 
-0.60 
-0.10 
-0.50 



Fifth 
sample 



+0.75 
+0.00 
-0.25 
+0.15 
+0.15 
+0.05 



+0.10 
-0.60 
-0.60 
-0.60 
-0.35 
-0.50 



TABLE 5 

Difference of ammonia and nitrates on unlimed soils compared with effect of treatment on lime 

requirement 



Humus soil: 

Ammonia (p.p.m.). 
Nitrates (p.p.m.) . . , 



Difference (p.p.m.) 

Difference in lime requirement 
(tons) 



302.4 
33.0 



+269.4 
-0.25 



268.8 
98.3 



+ 170.5 
-0.55 



98.0 
214.8 



•116.8 
+0.60 



86.8 
302.4 



-215.6 
+0.45 



32.0 
324.0 



-292.0 
+0.75 



Sandy soil: 

Ammonia (p.p.m.) 

Nitrates (p.p.m.) 

Difference (p.p.m.) 

Difference in lime requirement 
(tons) 



394.8 
9.4 


280.0 
112.2 


132.5 
167.9 


151.2 
267.6 


+385.4 
-0.50 


+ 168.8 
-45.0 


-35.4 
-0.20 


-116.4 
+0.10 



14.0 
312.4 

-298.4 
+0.10 



reduce rather than to increase it. A large production of ammonia reduces 
the lime requirement, and, quite logically, when nitrification occurred the 
opposite effect resulted. 



SOIL ACIDITY AND BACTERIAL ACTIVITY 



141 



Table 5 brings out this point when the cottonseed meal treatment is studied, 
in comparing the effect of ammonification and nitrification upon the decrease 
or increase of the lime requirement of the treated soil over the untreated. 

This table shows that though there is not a close correlation between the 
difference of ammonia and nitric acid produced on the soils treated with cotton- 
seed meal, and the effect upon the lime requirement, the tendency is for the 
soil to show a greater or smaller lime requirement according as there is more 
or less of the nitrogen present in the basic or acid form. None of the other 
treatments contain sufficient nitrogen to make the comparison significant. 

TABLE 6 
Residual carbonates on treated soils; expressed in tons per acre 



TREATMENT 



Humus soil: 

Soil alone 

Cottonseed meal 

Manure 

Dry timothy. . . . 

Dry clover 

Green timothy . . 

Green clover. . . . 
Sandy soil: 

Soil alone 

Cottonseed meal 

Manure 

Dry timothy. . . . 

Dry clover 

Green timothy. . 

Green clover. . . . 



3.40 
4.95 
4.10 
4.35 
4.15 
4.05 
4.20 

2.80 
3.90 
2.95 
3.25 
3.30 
2.80 
4.25 



< w 
5 w 



2.00 
1.20 
2.45 
2.35 
2.30 
2.50 
2.95 

2.45 
1.70 
2.65 
2.45 
2.75 
2.50 
3.00 



£2 



o ^ 



1.95 
1.25 
1.90 
2.10 
2.10 
2.30 
2.50 

2.55 
1.20 
2.60 
2.60 
2.90 
2.40 
3.00 



1.35 
0.55 
2.05 
1.90 
2.15 
2.15 
2.45 

2.40 
0.85 
2.40 
2.35 
2.50 
2.30 
2.85 



MORE OR LESS THAN SOIL UNTREATED 



First 
sample 



+ 1. 

+0 
+0 
+0 
+0 
+0 



+ 1.10 
+0.15 
+0.45 
+0.50 
+0.00 
+0.45 



Second 
sample 



+0.00 
+0.30 
+0.35 
+0.50 
+0.65 
+0.45 



-0.20 
+0.15 
+0.20 
+0.45 
+0.00 
+0.75 



Third 
sample 



-0.80 
+0.45 
+0.35 
+0.30 
+0.50 
+0.95 



-0.75 
+0.20 
+0.00 
+0.30 
+0.05 
+0.55 



Fourth 
sample 



-0.70 
-0.05 
+0.15 
+0.15 
+0.35 
+0.55 



-1.35 
+0.05 
+0.05 
+0.35 
-0.15 
+0.45 



Fifth 
sample 



0.80 
+0.70 
+0.55 
+0.80 
+0.80 
+ 1.10 



-1.55 
+0.00 
-0.05 
+0.10 
-0.10 
+0.45 



RESIDUAL CARBONATES 

The residual carbonates were determined by decomposing the remaining 
limestone with dilute acid, and titrating the carbon dioxide liberated, in the 
same way as the titration was made in the lime-requirement determinations. 
The results are given in table 6. 

Lime was applied at the rate of 7 tons on the more acid soil and 6 tons on 
the other soil, in the form of the precipitated carbonate. As was intended a 
sufficient excess was added so that nitrification did not exhaust it. 

The data show that in most cases the organic materials have tended to 
protect the lime applied to the soil. The notable exception is the cottonseed 
meal, which on account of the large production of nitric acid, has used up the 



142 



R. E. STEPHENSON 



carbonates nearly completely. All of the treatments helped to save limestone 
until nitrification occurred, as noted by the fact that with but three exceptions 
minus quantities do not appear until the last two samplings. 



SOLUBLE NON-PROTEIN NITROGEN 

The method employed in this study was in general that used by Potter and 
Snyder (4). The soil was extracted with 1 per cent hydrochloric acid, in both 
the limed and the unlimed series. The nitrate nitrogen and the ammonia 
nitrogen were distilled off by the Devarda reduction method. The residue 
from this reduction was then treated with sulfuric acid and the total nitrogen 
determined in the usual way. This latter gave the unknown soluble non- 
protein nitrogen of the acid extract. 

The acid-extracted soil was next extracted by shaking 2 hours with 1.75 
per cent sodium hydroxide, and the extract clarified by centrifuging for 5 

TABLE 7 
Soluble non-protein nitrogen in humus soil after 5 weeks 





UNKNOWN NON-PROTEIN NITROGEN 


TOTAL UNK 
PROTEIN I 


TOWN NON- 


TREATMENT 


In HC1 extract 


In alkalir 


e extract 


-flTROGEN 




No lime 


Lime 


No lime 


Lime 


No lime 


Lime 


Soil alone 


p. p.m. 

23.33 
195.99 
32.66 
28.66 
23.33 
35.33 
28.66 


p. p.m. 
26.00 
44.66 
11.33 
29.99 
26.00 
30.00 
19.60 


p. p.m. 

245.5 
310.5 
246.5 
232.0 
260.5 
266.0 
277.3 


p.p. m. 

246.5 
287.5 
218.0 
253.0 
244.0 
253.0 
253.3 


p. p.m. 

268.83 

506.49 

278.16 

260.66 

283.83 

301.33 

305.96 


p. p. m. 

272.50 


Cottonseed meal 


355.16 


Manure 


229.33 


Timothy 


282.99 


Clover 


270.00 


Green timothy 


283.00 


Green clover 


272.90 







minutes at 30,000 revolutions per minute. The extract was then neutralized 
with sulfuric acid, and acidified with tri-chlor-acetic acid sufficiently to give 
2\ per cent of the latter. The precipitate was then filtered off and another 
aliquot of the filtrate taken for determination of the nitrogen by the micro- 
method. 

Soluble non-protein materials should probably be the largest in amount when 
decomposition is the most active. But the question is, do these compounds, 
many of which are doubtless of a peptide character, tend to accumulate in 
soils in appreciable amounts, or do ammonification and nitrification proceed 
at once when the decomposition has started. In other words, should the solu- 
ble nitrogen be found primarily in the form of ammonia and nitrates or also 
in more complex forms? Previous study has shown that plants are capable 
of using more complex forms of nitrogen than nitrates and ammonia, and if 
they occur to any extent in ordinary soils, there may be conditions when such 
complex compounds function as direct sources of plant-food. 



SOIL ACIDITY AND BACTERIAL ACTIVITY 143 

The results show in every case but one (timothy) that the application of 
lime has diminished the total unknown soluble non-protein nitrogen. The 
nitrates and ammonia, though soluble non-protein nitrogen, are not included 
in these data. A reference to table 3 shows that this is the same general 
tendency as observed in the production of ammonia. There is one noticeable 
fact, and that is that none of the organic treatments have as marked an effect 
upon the amount of unknown soluble non-protein nitrogen as they have on the 
nitrates and ammonia. This indicates, as do also the data of Potter and Sny- 
der (4), that in the decomposition of proteins of the soil the degradation prod- 
ucts undergo rather rapidly a complete change to the simpler state of ammonia, 
and nitrate. Except in case of the more resistant forms, possibly polypeptides 
of some degree of complexity, the products apparently do not accumulate to 
a large extent, and the nitrogen of the soil must exist mostly as the more 
complex and resistant forms or else as the simplest possible products of de- 
composition. Ordinarily, of course, nitrates and ammonia are removed from 
the soil almost as rapidly as produced, and therefore they are not found in 
large amounts at any one time. Hence the soluble non-proteins such as 
are found in this study are probably present at any definite time in perhaps 
five or even ten times the amount of ammonia and nitrates present. 

Another question to consider is the possible effect of such compounds on 
the reaction of the soil. Though perhaps capable of reaction as either acids 
or bases, they are not found in sufficient quantity to exert a marked effect upon 
soil reaction. Such materials and others, however, doubtless exercise a buffer- 
ing effect and help to reduce the hydrogen-ion concentration to some extent. 

GENERAL DISCUSSION 

This experiment was continued for 159 days, or about 22 weeks. It is 
not presumed that there would be no change after this time, but rather that 
such changes as occurred previous to this would determine whatever effects 
were to be produced by the different treatments on the activity of soil organ- 
isms or the reaction of the soil. 

The materials used contained the following percentages of nitrogen: dry 
timothy, 0.693; manure, 1.24; green timothy, 1.28; dry clover, 2.30; green 
clover, 2.82; and cottonseed meal, 6.96 per cent. The poorer soil contained 
0.116, and the better soil about twice as much, or 0.238 per cent, of nitrogen. 
The amounts of nitrogen found as ammonia and nitrates were for the most 
part in the same order as the percentages of nitrogen contained in the mate- 
rials used. 

No definite conclusions may be drawn from a limited study, but in general 
it seems that the essential soil organisms are active in soils of at least moder- 
ately strong acidity. The data indicate also that the decay of organic mate- 
rials under aerobic conditions does not produce an appreciable acidity except 
where nitric acid is formed in nitrification. 



144 R. E. STEPHENSON 

SUMMARY 

1. The lime requirement of neither soil was increased by the organic treat- 
ments except in those cases where there was a large production of nitric acid. 

2. Ammonification is apparently greater in the absence of lime, partly per- 
haps because of the fact that nitrifying organisms have been less active. 

3. Lime has generally stimulated nitrification. 

4. The sum of ammonia and nitrates is usually greater on the unlimed 
soil when treated with nitrogenous organic materials. This is doubtless partly 
due to the increased number of organisms in the presence of lime and the 
consequent greater consumption of nitrates and ammonia by the organisms. 

5. When nitrogenous sources of energy such as horse manure and timothy 
hay were supplied, nitrifiction and ammonifiction were reduced below that of 
the untreated soil. 

6. The green materials were somewhat more readily attacked than the dried 
materials. There was greater production of ammonia and nitrates partly 
however because of the fact that these materials were richer in nitrogen than 
the mature plants. 

7. The soluble unknown non-protein nitrogen determined at the second 
sampling on the more fertile soil, when the activity of the organisms was nearly 
at the maximum, showed little effect due to the various organic treatments. 
The cottonseed meal was the only treatment which gave any large increase 
over the untreated soil. 

8. In all cases but one, the unlimed treatments gave a higher non-protein 
nitrogen content than the limed. 

REFERENCES 

(1) Davis, C. W. 1917 Studies on the phenol-disulphonic acid method for determining 

nitrates in soils. In Jour. Indus. Engin. Chem., v. 9, no. 3, p. 290. 

(2) Miller, M. F. 1917 Effect of the addition of organic matter to the soil upon the 

development of soil acidity. In Mo. Agr. Exp. Sta. Bui. 147, p. 50-51. 

(3) Skinner, J. J., and Beattie, J. H. 1917 Influence of fertilizers and soil amend- 

ments on soil acidity. In Jour. Amer. Soc. Agron., v. 9, no. 1, p. 25-35. 

(4) Snyder, R. S., and Potter, R. S. 1918 Soluble non-protein nitrogen of the soil. In 

Soil Sci., v. 6, p. 441-448. 

(5) Stephenson, R. E. 1918 Soil acidity methods. In Soil Sci., v. 6, p. 33-52. 

(6) Stephenson, R. E. 1918 The effect of organic matter on soil reaction. In Soil Sci., 

v. 6, p. 413-439. 

(7) Stephenson, R. E. 1919 The activity of soil acids. In Soil Sci., v. 8, p. 41-59. 

(8) White, J. W. 1918 Soil acidity and green manures. In Jour. Agr. Res., v. 13, no. 3, 

p. 171-197. 



Reprinted from Soil Science 
Vol. XII, No. 2, August, 1921 



THE EFFECT OF ORGANIC MATTER ON SOIL REACTION. IP 

R. E. STEPHENSON 

Agricultural College, University of Kentucky 

Received for publication February 17, 1921 

INTRODUCTION 

A study on the effect of organic matter on soil reaction was undertaken as a 
part of an extended investigation of soil acidity. For a description of the 
background of the experiments here reported, experimental methods, etc., 
the reader is referred to the preceding study (5), also to a former study of the 
same problem (4). 

In this series of treatments the organic materials were applied at the same 
rates as before (10 tons) (4) except where dried blood and straw were mixed 
and then blood was used at the rate of 10 tons, with 5 and 10 tons of straw. 
Precipitated carbonate of lime was added to the limed treatments at the uni- 
form rate of 5 tons per acre. The materials used were soybean hay, green 
rape, oat straw, green soybean hay (pods removed), dried blood and a mixture 
of blood and oat straw, all in both the limed and the unlimed series. The 
green materials were dried, as were also the other materials, and ground as 
finely as was convenient before adding to the soil. The soil used in this study 
was an acid silt loam taken from the West Virginia Agricultural Experiment 
Station farm, rather heavy and compact, and poor in organic matter. 

The total period of incubation was 125 days, samplings being made at 
intervals of 2, 5, 10 and 18 weeks, respectively. In addition to the determina- 
tions made in the study of the previous series, hydrogen-ion determinations 
were made upon all treatments. 

AMMONIFICATION 

The aeration method was again used for ammonia. The results are shown 
in table 1, expressed as parts of nitrogen per million of soil. 

Only the blood possessed a high nitrogen content and therefore it is the 
only material which caused a large production of ammonia. 

1 This paper is the second part of a thesis presented to the graduate faculty of the Iowa 
State College of Agriculture in partial fulfillment of the requirements for the degree of 
Doctor of Philosophy. It is also the second paper published on this study, the former (4) 
having appeared in 1919. A portion of the work here reported was completed at the Iowa 
Agricultural Experiment Station, and the remainder was conducted in consultation with 
Prof. R. M. Salter at West Virginia University. Acknowledgments are extended to Dr. 
P. E. Brown, of Iowa State College, and also to Professor Salter, for helpful suggestions in 
planning and interpreting the work. 

145 



146 



R. E. STEPHENSON 



Lime produced no marked effect in the ammonification of any of the ma- 
terials until the third sampling, when it caused an appreciable reduction, which 
was still very evident at the last sampling. This may have been due to two 
causes. The lime may have caused greater numbers of organisms to grow, 
which in turn caused a greater consumption of ammonia. The principal 
cause, no doubt, was that lime permitted greater nitrification, and most of 
the ammonia had been changed over to nitrate. The data show that this 
had occurred. 

The oat straw depressed ammonification just as it did nitrification, in most 
cases below that of the untreated soil; this would indicate that it was a suitable 
source of energy for bacterial activity. 

Green soybeans likewise depressed ammonification below that of the soy- 
bean hay but partly because of the fact that their nitrogen content was lower. 

TABLE 1 
Amount of ammonia at the end of each period 



TREATMENT 



Silt loam soil: 

Soil alone 

Soybean hay 

Green rape 

Green soybeans 

Oat straw 

Blood 

Blood and 5 tons of straw. . 
Blood and 10 tons of straw 

Averages 



FIRST SAMPLE, 
2 WEEKS 



No 
lime 



Lime 



p. p.m. 

3.6 
94.0 
182.0 

48.0 

52.0 

342.0 

242.0 

226.0 



169.4 



p. p.m. 

50.0 
106.0 
168.0 
63.0 
20.0 
282.0 
316.0 
300.0 



180.0 



SECOND SAM- 
PLE, 5 WEEKS 



THIRD SAMPLE, 
10 WEEKS 



No 
lime 



p. p.m. 

32.0 
107.0 
178.0 
40.0 
24.0 
566.0 
424.0 
396.0 



247.9 



Lime 



p.p.m 

34.0 
106.0 
132.0 
56.0 
16.0 
425.0 
400.0 
306.0 



209.7 



No 
lime 



p.p.m. 

60.0 
92.4 
36.4 
16.9 

13.8 
546.0 
336.2 
288.4 



190.1 



Lime 



p.p.m. 

5.6 

8.7 

8.7 

22.2 

5.6 

361.2 

43.2 

26.8 



68.1 



FOURTH SAM- 
PLE, 18 WEEKS 



No 
lime 



p.p.m. 

16.0 

36.0 

64.0 

12.0 

10.4 

328.0 

440.0 

366.0 



179.5 



Lime 



p.p.m 

12.0 

8.0 

4.0 

8.0 

8.0 

54.0 

48.0 

32.0 



23.1 



No 
lime 



p.p.m. 

36.0 

82.4 

115.1 

29.2 

25.1 

445.5 

360.5 

319.1 



196.7 



Lime 



p.p.m. 
2.54 

57.2 

78.2 

38.5 

12.4 

287.3 

201.8 

166.2 

120.2 



Green rape, on the other hand, stimulated ammonification next to the dried 
blood. However, it contained a little less nitrogen than the soybean hay, 
though more than the green soybeans. 

Straw mixed with blood had little consistent effect upon ammonification. 
However, the ammonia produced by the combined application of blood and 
straw was seldom greater and often less than that produced from the blood 
alone. 

There were individual cases where the limed treatments produced more 
ammonia than the unlimed, but when averages of all treatments (omitting 
the soil alone) and of all samples, were taken, the unlimed treatments have 
produced a greater quantity of ammonia. The difference is quite marked at 
later samplings when nitrification is well under way. 

The data show that the accumulation of nitrates has increased at each 
successive sampling with all treatments, as well as with the untreated soil, 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: II 



147 



in both the limed and the unlimed series. In general, there has been greater 
nitrification in the presence of lime. This is more noticeable after the first 
sampling and with the nitrogen rich materials. Lime apparently had the 
opposite effect where oat straw was used. Straw used with blood retarded 
nitrification at first but later there was little or no retardation. The maximum 
amount of nitrates occurred at the last sampling in most cases. 

Apparently the green soybeans began to nitrify more quickly than did the 
soybean hay. Green rape likewise at once stimulated nitrification to an 
appreciable extent. 

NITRIFICATION 

Nitrates were determined by the colorimetric method as before. The 
results are shown in table 2. 

TABLE 2 

Nitrates at each successive sampling 



TREATMENT 



Silt loam soil: 

Soil alone 

Soybean hay 

Green rape 

Green soybeans 

Oat straw 

Dried blood 

Blood and 5 tons of straw. 
Blood and 10 tons of straw. 



Average 26.7 22.7 



FIRST SAMPLE, 
2 WEEKS 



No 
lime 



p. p.m. 

19.6 
25.9 
76.1 
42.5 
19.8 
14.2 
8.6 
Tr. 



Lime 



p. p.m. 

23.0 

25.7 
96.6 
27.2 
Tr.* 
9.1 
Tr. 
Tr. 



SECOND SAM- 
PLE, 5 WEEKS 



THIRD SAMPLE 
10 WEEKS 



No 
lime 



p. p.m. 

24.8 
28.8 
79.0 
388.5 
19.6 
24.5 
17.3 
59.1 



Lime 



p. p.m. 

33.3 
37.9 
75.1 
39.7 
4.6 
40.7 
37.1 
59.8 



No 
lime 



p. p.m. 

38.4 
71.8 
120.5 
83.5 
26.3 
149.1 
160.3 
156.9 



Lime 



p. p.m. 



54 
181 
234 
108 

16 
485 
280 
492 



FOURTH SAM- 
PLE, 18 WEEKS 



No 

lime 



p. p.m. 

65.3 
122.6 
260.3 
175.7 

38.6 
353.3 
332.7 
413.0 



p. p.m. 



Lime 



113 
195 
188 
109 
38 
611 
640 
575 



2 42.1 109.8271.5242.3J336.9 116.7 168.3 



No 
lime 



p. p.m. 

37.0 
62.3 
133.9 
172.5 
26.1 
135.3 
129.8 
157.2 



Lime 



p. p.m. 

56.2 
110.0 
148.6 
71.0 
15.4 
286.6 
264.5 
281.9 



Tr. = trace. 



Nitrification apparently scarcely occurred in the presence of oat straw 
until the third and fourth samplings. In no case was there as much nitrate 
as on the untreated soil. 

Nitrification was slow in starting when blood and straw were mixed but 
by the end of 10 weeks there was an appreciable accumulation of nitrates on 
the treated soils over the untreated. Apparently the addition of straw had 
no marked effect upon the nitrification of dried blood. 

When averages of all treatments and all samplings are taken (omitting 
the untreated soil) it is observed that nitrification was slow in starting where 
straw and blood and mixtures of the two were used, but the blood-straw 
mixtures finally ran high. Lime in these cases seems to have retarded the 
beginning of the nitrifying process, but perhaps more organisms were present 
where lime was added and they were consuming such nitrates as were produced. 



148 



R. E. STEPHENSON 



The nitrogen summary shown in table 3 indicates that the average total of 
nitrates and ammonia has been greatest in most cases for the treated soils, 
when not limed, but that the reverse is true for the untreated soil. Whether 
the difference may be due to numbers of organisms and the consequent utili- 
zation of part of the nitrogen changed on treated limed soils, cannot be stated, 
though it seems probable. Experience has shown that in nearly every case a 
carbohydrate material such as straw which is poor in nitrogen, has given a 
decrease in ammonia and nitrates over the soil alone, either limed or unlimed. 
Since the ammonia and nitrate forms of nitrogen are by-products of the 
attempt of the organism to secure sufficient energy, this is to be expected. 

TABLE 3 
Nitrogen summary, nitrates and ammonia 





FIRST SAMPLE, 
2 WEEKS 


SECOND SAM- 
PLE, 5 WEEKS 


THIRD SAMPLE, 
10 WEEKS 


FOURTH SAM- 
PLE, 18 WEEKS 


AVERAGE 




No 
lime 


Lime 


No 
lime 

p. p.m. 


Lime 


No 
lime 


Lime 


No 
lime 


Lime 


No lime 


Lime 




p. p. m. 


p. p.m. 


p. p. m. 


p. p.m. 


p. p.m. 


p.p m. 


p. p.m. 


p. p.m. 


Minus 


p. p.m. 


Minus 


Silt loam soil: 




















soil 




soil 


Soil alone. . . 


55.6 


73.0 


57.3 


67.3 


98.4 


59.9 


81.3 


125.8 


73.1 


p. p.m. 


81.5 


p. p.m. 


Soybean 


120.0 


131.8 


130.8 


144.0 


164.2 


189.8 


158.6 


203.3 


143.4 


70.3 


167.2 


85.7 


Green rape. . 


258.1 


264.6 


257.0 


207.1 


156.9 


242.8 


324.3 


192.7 


249.1 


176.0 


226.8 


145.3 


Green 


























soybeans . 


90.5 


95.2 


78.5 


95.7 


100.4 


130.4 


187.7 


117.0 


114.3 


-41.2 


109.8 


28.3 


Oat straw. . . 


71.8 


20.0 


43.6 


20.6 


40.1 


23.8 


49.1 


46.9 


51.2 


-21.9 


27.8 


-53.7 


Dried blood. 


356.2 


291.1 


590.6 


492.7 


745.1 


846.7 


681.1 


665.1 


580.8 


507.7 


548.9 


467.4 


Blood and 5 


























tons of 


























straw .... 


250.6 


316.0 


441.8 


437.3 


496.5 


424.1 


777.7 


687.9 


491.7 


418.6 


475.8 


394.3 


Blood and 


























10 tons of 


























straw .... 


266.0 


300.0 
202.7 


455.0 


365.8 


445.3 


519.2 


779.0 


607.4 


477.2 


404.1 


488.1 


366.6 


Average. . . . 


196.1 


336.1 


251.8 


299.9 


339.6 


421.8 


360.0 


301.1 




286.3 





LIME REQUIREMENT 

The data show that in nearly every case the lime requirement was less when 
organic matter was added to the soil (table 4). The greatest effect was usually 
at the first sampling. This was especially marked with the dried blood which 
produced large amounts of ammonia. Next to blood, soybean hay produced 
the greatest effect; green rape was next and oat straw last. Thus it seems 
that nitrogenous materials, by their production of ammonia and perhaps by 
other reactions, reduce the lime requirement of soils. The effect has been 
more marked and consistent on this rather heavy soil than on the lighter 
soils previously studied. Carbohydrate materials have much smaller effects. 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: II 



149 



It is shown also that the limed soils have a capacity for decomposing lime- 
stone, even after 18 weeks' standing with an excess of lime. This would indi- 
cate that acid soils react with carbonate of lime beyond the neutral point, or 
that for lack of sufficiently intimate contact, all the acids have not yet been 
neutralized. There is perhaps no such thing as completion of the reaction. 
There are doubtless always soluble acids or acid salts capable of decomposing 
the carbonate. 

TABLE 4 
Lime requirement of variously treated soils (tons per 3,000,000 pounds) 



Clay soil: 

Soil alone 

Soil limed 

Soybean hay 

Limed 

Green rape 

Limed 

Green soybeans 

Limed 

Oat straw 

Limed 

Blood 

Limed 

Blood and 5 tons of straw . . 
Limed 

Blood and 10 tons of straw. 
Limed 



FIRST 
SAMPLE, 

2 

WEEKS 



3.35 
0.95 

2.00 
0.35 

2.10 
0.60 

1.85 
0.45 

2.65 
0.45 

2.00 
0.35 

1.85 
0.35 

1.70 
0.40 



SECOND 
SAMPLE 

5 
WEEKS 



2.95 
0.55 

2.60 
0.95 

2.50 
0.80 



THIRD 
SAMPLE, 

10 
WEEKS 



3.10 
0.45 

2.60 
0.60 

2.65 
0.55 



FOURTH 
SAMPLE, 

18 
WEEKS 



2.85 


2.65 


0.95 


0.55 


2.60 


2.65 


0.75 


0.75 


1.80 


2.00 


0.90 


0.65 


2.65 


1.90 


1.05 


0.80 


2.20 


2.05 


1.20 


0.90 | 



3.10 
0.65 

3.10 
0.80 

3.20 
0.90 

3.00 
0.60 

2.85 
0.65 

2.95 

1.35 

3.05 
1.50 

3.00 
1.40 



MORE OR LESS THAN SOIL ALONE 



First Second Third Fourth 
sample sample sample sample 



-1.35 
-0.60 



-0.35 
+0.40 



-1.25-0.45 
-0.35+0.25 



-1.50 
-0.50 

-0.70 
-0.50 

-1.35 
-0.60 

-0.50 
-0.60 

-0.65 
-0.55 



-0.50 
+0.15 

-0.45 
+0.10 



-0.10-0.45 
+0.40+0.10 



-0.35 
+0.30 

-1.15 
+0.35 

-0.30 
+0.50 

0.75 
+0.65 



-0.45 
+0.30 

-1.10 
+0.20 

-1.20 
+0.35 

1.05 
+0.45 



+0.00 
+0.15 

+0.10 
+0.25 

-0.10 
-0.05 

-0.25 
+0.00 

0.15 
+0.70 

0.05 
+0.85 

-0.10 

+0.75 



It is worthy of note, too, that the organic treatments seem to have increased 
the capacity of the soil to react with lime, when they were used alone. 

There is a rather close correlation between changes in soil reaction, and the 
nitrogen changes as shown by table 5. This is especially noticeable on the 
blood treatments where there is sufficient nitrogen added to produce a measur- 
able effect upon the reaction. 

These data show a close correlation between the excess of ammonia over 
nitrates and the true acidity, or pH values of the soils, and would signify 
that the bacteriological changes which were occurring were affecting the soil 
reaction to an appreciable extent. 



150 



R. E. STEPHENSON 



The same thing is shown in table 6 on all treatments, considering the sum- 
marized effects as before. 

TABLE 5 

Difference of ammonia and nitrates on unlimed soils compared with the effect of the treatment 

on soil reactions 



SILT LOAM SOIL 



Blood treatment only 

Ammonia (p.p.m.) 

Nitrates (p.p.m.) 

Excess (NH 3 ) (p.p.m.) 

pH on blood 

pH increase over untreated soil 



FIRST 
SAMPLE 



270.0 

7.8 



+262.2 
6.33 
+ 1.62 



SECOND 
SAMPLE 



462.0 
33.8 



+428.0 
7.00 
+2.12 



THIRD 
SAMPLE 



390.0 
155.4 



+234.6 
6.46 
+1.68 



FOURTH 
SAMPLE 



378.0 
366.3 



+ 11.7 
5.41 
+0.54 



TABLE 6 
Nitrogen changes and the effect on soil reaction summarized 



ALL TREATMENTS NO LIME 



Ammonia (p.p.m.) 

Nitrates (p.p.m.) 

Difference (p.p.m.) 

pH values 

pH increase over untreated soil 



FIRST 
SAMPLE 


SECOND 
SAMPLE 


THIRD 
SAMPLE 


169.4 

26.7 


247.9 

88.2 


190.1 
109.8 


+ 142.7 
6.01 
+ 1.10 


+ 159.7 
6.23 

+ 1.35 


+80.3 
5.97 
+ 1.19 



FOURTH 
SAMPLE 

179.5 
242.3 

-62.8 
5.17 
+0.29 



TABLE 7 
Residual carbonates on treated soils at the various samplings, expressed as tons per acre 



Clay soil: 

Soil alone 

Soybean hay 

Green rape 

Green soybeans 

Oat straw 

Blo6d 

Blood and 5 tons of straw . 
Blood and 10 tons of straw 



FIRST 
SAMPLE 



2.35 
3.15 
3.15 
3.25 
3.20 
3.15 
3.70 
3.60 



SECOND 
SAMPLE 



2.20 
2.40 
2.10 
2.00 
1.85 
3.05 
3.20 
3.20 



THJRD 
SAMPLE 



1.40 
1.05 
1.20 
1.45 
1.40 
0.60 
1.05 
0.70 



FOURTH 
SAMPLE 



0.90 
1.05 
0.45 
1.00 
0.95 
0.00 
0.10 
0.00 



MORE OR LESS THAN SOIL ALONE 



First Second Third Fourth 
sample sample sample sample 



+0.60 
+0.60 
+0.70 
+0.65 
+0.60 
+ 1.15 
+ 1.05 



+0.20 
-0.10 
-0.20 
-0.35 

+0.85 
+ 1.00 
+ 1.00 



-0.35 
-0.20 
+0.05 
+0.00 
-0.80 
-0.35 
-0.70 



+0.15 
-0.45 
+0.10 
+0.05 
-0.90 
-0.90 
-0.90 



RESIDUAL CARBONATES 



The data show that the organic matter protected the carbonates until there 
was considerable nitrification. All organic treatments caused a marked sav- 
ing of carbonates at the first sampling. At the last sampling those treatments 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: H 



151 



where there was much nitrogen to produce nitric acid, nearly or completely 
exhausted the carbonates present. Even the untreated soil reacted slowly 
and continually and would perhaps have used up all the limestone after suffi- 
cient time, even though there was no leaching. 

These data would indicate that excessive nitrification might become a positive 
factor in contributing to soil acidity. However, nitrates, being soluble, will 
not accumulate and in the process of leaching basic material is permanently 
removed from the soil. 



HYDROGEN-ION CONCENTRATION 



The hydrogen-ion concentration was determined at each sampling on all 
of the treatments with the hydrogen electrode apparatus. 



TABLE 8 
Hydrogen-ion concentration 



Clay soil 

Soil alone: 

Soil and 5 tons of lime 

Soybeans and straw 

Soybeans and lime 

Green rape 

Green rape and lime 

Green soybeans 

Green soybeans and lime 

Oat straw 

Oat straw and lime 

Blood 

Blood and lime 

Blood and 5 tons of straw 

Blood, 5 tons of straw and lime 

Blood and 10 tons of straw . . . 

Blood, 10 tons of straw and lime 



FIRST 
SAMPLE 



pE 
4.91 

7.62 
6.03 
7.74 
6.03 
7.66 
5.81 
7.74 
5.21 
7.48 
6.48 
7.71 
6.28 
7.74 
6.24 
7.65 



SECOND 
SAMPLE 



PE 



7.72 
5.89 
7.64 
6.05 
7.65 
5.58 
7.64 
5.07 
7.60 
7.17 
7.91 
7.10 
7.76 
6.74 
7.76 



THIRD 
SAMPLE 



PH 
4.78 

7.65 
5.50 
7.41 
6.17 
7.42 
5.34 
7.60 
5.41 
7.66 
6.55 
7.22 
6.58 
7.34 
6.24 
7.36 



FOURTH 
SAMPLE 



PB 



7.60 
5.02 
7.51 
4.78 
7.53 
5.17 
7.74 
5.00 
7.71 
5.43 
7.60 
5.38 
7.54 
5.44 
7.61 



MORE OR LESS THAN SOIL ALONE 



First Second Third Fourth 
sample sample sample sample 



pH 



+ 1.12 
+0.12 
+ 1.12 
+0.04 
+0.90 
+0.12 
+0.30 
-0.14 
+1.57 
+0.09 
+ 1.37 
+0.12 
+ 1.33 
+0.03 



PH 



+ 1 

-0 

+1 

-0 
+0 
-0 
+0 
-0 
+2 
+0 
+2 
+0 
+ 1 
+0 



PE 



+0 
-0 

+1 

-0 

+0 
-0 

+0 
+0 

+1 

-0 

+1 

-0 

+1 

-0 



PE 



+0.14 
-0.03 
-0.10 
-0.07 
+0.29 
+0.14 
+0.12 
+0.11 
+0.55 
+0.00 
+0.50 
-0.06 
+0.56 
+0.01 



The lime requirement according to the Tacke method was a little more than 
3 tons. To take care of acids which might be produced in the decomposition 
of organic material, an excess of 2 tons was used. The data (table 8) show 
that this was sufficient to give a slightly alkaline soil either with or without 
organic treatment (the smaller the pH value the more acid the soil). Every 
organic treatment without lime diminished the true acidity of the soil, the 
highly nitrogenous materials most, as was true also of the lime requirement. 
The oat straw had the least effect. In the presence of lime, however, the 
organic treatments had a rather slight effect in reducing the hydrogen-ion 
concentration at first, and by the third sampling the effect was the reverse in 



152 R. E. STEPHENSON 

nearly every case, though again the increase in hydrogen-ion concentrations 
was not large. By the fourth sampling the effects were quite erratic. In 
nearly every case where lime was not used, however, the organic treatments 
reduced the acidity somewhat. 

GENERAL DISCUSSION 

The materials used in this study were such as are common crop residues 
or fertilizers. The nitrogen content was: oat straw, 1.05; green soybeans, 
2.41; green rape, 3.43; soybean hay, 6.63; and dried blood, 13.93 per cent. 
The 5 -ton application of limestone proved to be scarcely enough to take care 
of the natural soil acidity plus that produced in nitrification as shown by the 
data. 

The lime requirement shown by the Tacke method on this soil was about 
3 tons. Shaking and aeration was continued for only 5 hours, however, in 
this and the remaining work, partly for convenience and partly because of 
the fact that a limited amount of work had shown that the lime requirement 
indicated by a 5-hour run was sufficient. When that quantity of lime was 
added to the soil and allowed to stand for a short time with optimum moisture 
conditions, a practically neutral reaction was shown by hydrogen-ion 
determinations. 

The results of the effect of carbohydrate materials upon nitrification have 
a practical bearing which is worthy of consideration. Experience has shown 
that the plowing under of green manures such as rye, the heavy use of straw, 
and other refuse, often cause disappointing yields from the crop immediately 
following. This may result not alone because the crop has exhausted the 
water supply previous to plowing under, but oftentimes no doubt, because 
such materials have furnished the soil organisms with easily available sources 
of energy, and nitrification does not proceed rapidly enough to supply the crop 
with nitrates. Thus the immediate crop suffers nitrogen starvation, though 
perhaps later crops might be much benefited. 

SUMMARY 

1. Oat straw again reduced nitrification and ammonification below that of 
the untreated soil. 

2. A mixture of straw and blood reduced the total nitrogen found in the 
form of ammonia and nitrates below that of the blood treatment alone. Ten 
tons of straw with the blood caused a somewhat greater reduction than the 
5-ton application. 

3. All the treatments reduced the lime requirement indicated by the Tacke 
method, until nitrification had taken place. 

4. Lime-requirement determinations of the limed soils showed that the 
treated soils were always capable of reaction with more lime, though an 
excess of 2 tons of limestone had been applied. This shows that the soils 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: II 



153 



contain acids which are very slowly reactive, and perhaps they will react with 
limestone beyond their neutral point. 

5. The residual carbonates, where blood was applied, were completely 
exhausted at the last sampling. 

6. The hydrogen-ion determinations show that in practically every case 
the organic treatments reduced the true acidity. In some cases, on the 
contrary, both lime and organic treatments did not give as alkaline a soil as 
did the lime alone. 

7. Changes in soil reaction especially on the blood-treated soils, follow 
very closely the deficit or excess of ammonia over nitric nitrogen, indicating 
that these processes may become factors influencing the production of acid 
soils. 

BUFFERING IN SOILS 

Practically all soils possess perhaps some degree of buffering, that is, they 
are able to react with either base or alkali to a certain extent, without very 

TABLE 9 

Table showing treatments and the hydrogen-ion concentration increments corresponding 



son. 


ALONE 


1000 

POUNDS 

Ca(OH)j 


2000 
POUNDS 

Ca(OH)j 


4000 

POUNDS 

Ca(OH> 


8000 

POUNDS 

Ca(OH) 2 


16,000 

POUNDS 

Ca(OH) 2 


32,000 

POUNDS 

Ca(OH)i 


A. Muck 


PH 

4.5 

5.1 

5.0 

5.8 

4.6 

7.0 

7.9 


pH 

0.2 

0.1 

0.2 

0.6 

0.1 

0.3 

0.3 


pH 

0.3 

0.3 

0.2 

0.9 

0.1 

0.3 

0.0 


m 

0.2 

0.7 
0.7 
0.5 
0.3 
0.3 
0.0 


PR 

0.6 

1.0 

0.8 

0.4 

0.9 

0.3 

0.2 


PH 

0.3 

1.0 

0.7 
0.3 
1.2 
0.2 
0.1 


PH 
7 


B. Fine sand 


1 


C. Red clay 


9 


D. Coarse sand 

E. Mucky loam 

F. Neutral soil 


0.8 
0.5 







much change in hydrogen-ion concentration. The degree of buffering and the 
rate of change of reaction with increasing amounts of base or acid will depend 
very much upon soil type, as the following data will show. 

Five grams each of the soils listed in table 9 were treated with 0.02 N 
Ca(OH) 2 equivalent to the various amounts of lime per acre of 2,000,000 
pounds of soil, evaporated to dryness on the steam bath, taken up with 20 cc. 
of water, allowed to stand over night, and the hydrogen-ion concentration 
determined with a hydrogen-electrode apparatus. The acid-treated soils 
were managed in the same way, 0.008 N H 2 S0 4 being used. 

The tabulated data show that the rate of change of reaction with increasing 
increments of lime is very different for the different soils. The muck soil 
shows the highest buffering and the sand the least, as would be expected. 
The neutral and alkaline soils do not change very greatly, showing that they 
have little capacity for buffering against a base. 

The data in table 10 show the effect of acid treatments. 



154 



R. E. STEPHENSON 



As was true of buffering against bases, the organic soils show a greater 
capacity for buffering against acids. The sandy soil shows less buffering, and 
the neutral and alkaline soils have great apparent buffering power, perhaps 
due to the presence of excess bases. 

In general, mucky or organic soils should show the highest degree of buffer- 
ing, clays less, and sands the least. The protein materials of the organic 
soils, and the acid silicates of clayey soils are doubtless responsible for most 
of the buffer action of such types. Sands, containing perhaps little of either, 
are not usually highly buffered. 

The highly buffered soils should show not only less change with the first 

treatments of base or acid but should continue to resist change of reaction 

longer when larger treatments are given. The initial reaction, of course, will 

be a factor to consider at this point. But it is worthy of note that the soils 

A and C, which are most acid to start with, show the greatest capacity for 

buffering against acid. 

TABLE 10 

Treatments and corresponding H-ion concentrations by increments corresponding to treatments 

16,000 

POUNDS 

H 2 SO. 

pn 
0.4 
0.4 
0.4 
0.3 
0.3 
2.0 
0.0 



A. Muck 

B. Fine sand. .. 

C. Red clay 

D. Coarse sand. 

E. Mucky loam 

E. Neutral 

G. Alkaline 



ALONE 


1000 

POUNDS 

H 2 S0 4 


2000 
POUNDS 

H 2 SOi 


4000 

POUNDS 

H 2 SO« 


8000 

POUNDS 

H 2 SO 


pn 


pa 


PH 


pn 


PH 


4.5 


0.4 


0.2 


0.2 


0.2 


5.1 


0.3 


0.3 


0.5 


0.5 


5.0 


0.4 


0.2 


0.4 


0.5 


5.7 


1.0 


0.4 


0.8 


0.4 


4.5 


0.3 


0.3 


0.3 


0.3 


7.0 


0.1 


0.1 


0.3 


0.3 


7.9 


0.1 


0.0 


0.2 


0.0 



It might be supposed that since soils tend naturally to become acid the 
capacity for buffering against acids would be more or less exhausted. It is 
demonstrated that this is true to a limited extent only. While the first appli- 
cation of acid causes a comparatively large change in reaction, it is observed 
that there is a marked buffering which continues to be manifested with the 
highest treatments. The acid soils likewise, however, have a greater capacity 
for base buffering. 

These facts are best brought out by means of graphs (fig. 1), which show the 
rate of change by the degree of curvature. Soil A has a curve much less steep 
than the other soils, soil D having much the most abrupt slope. The acid 
curves for B and D reach a final point nearly together, though starting quite 
widely separated. Soil A , which is by far the most acid, never rises to as high 
an acidity as soil D, which is by far the least acid. Soil A is a muck, while 
soil D is a sand and this difference in buffering capacity could be predicted, 
though such an extreme effect may seem extraordinary. 

The above data have considerable significance in various ways. They 
demonstrate what practical experience has already indicated, that soils may 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: II 



155 



be quite acid when the total lime requirement is measured, and yet have a 
comparatively low active acidity. Ordinary soil-acidity methods measure 
the capacity of the soil for decomposing lime rather than its true acidity or 
hydrogen-ion concentration. Soils high in organic matter may be able to 
take up large amounts of limestone, when a great part of this acidity has been 
overshadowed by amphoteric substances. 

Highly buffered soils also may permit vigorous bacterial activity, because 
the buffering effect keeps down the hydrogen-ion concentration to a point 




1000 lbe.2000 lbs. 4000 lbs. 



8000 lbs. 
Pounds of treatment 



Fig. 1. Titration Curves for Soils A, B, C, and D, with Different Amounts of 

Ca(OH) 2 and H 2 S0 4 Added 
Ca(OH) 2 Curve— upper graph 
H2SO4 Curve — lower graph 

which is not destructive of the soil organisms. A soil on the other hand which 
is not buffered, has a higher hydrogen-ion concentration though a smaller 
total lime requirement, and organisms are not very active because of the 
deleterious effects of the unbuffered acids. 

The importance of hydrogen-ion concentration biologically may be shown 
by the following data taken from Fred's work (2) with legume bacteria. 
Only the acid limits are given, but perhaps the alkaline limit would be nearly 
as far above neutrality, which would mean a wide variation for some organ- 
isms and only a narrow one for others. 



156 R. E. STEPHENSON 

arid 
limit 
PB 

1. Alfalfa and sweet clover 5.0 

2. Garden pea, field pea, vetch 4.8 

3. Red clover and common beans 4.3 

4. Soybeans and velvet beans 3.4 

5. Lupines 3.2 

6. Limits of growth of Azotobacter about 6 . 6 to 8 . 8 

Many soil organisms are even more sensitive to reaction than some of the 
common legume organisms, and thus the true acidity of soils is doubtless the 
determining factor for the biological changes which are to occur. No data 
can be given here for the reaction permitting mold growth, but it is known 
that they endure high degrees of acidity and probably no soil under ordinary 
treatment is ever too acid for their activity. 

THE NATURE OF SOIL ACIDITY 

Hydrogen-ion studies 

In the following brief study tumblers of soil treated in various ways were 
used to determine the effect of the treatment upon the hydrogen-ion con- 
centration. In each case 100 gm. of dry soil were employed, and the moisture 
content kept at the optimum (50 per cent of saturation). One series was 
treated with ammonium sulfate at the rate of 1 ton per acre, and lime in 
increasing increments, 1, 3, 5, 7, 9, 12, and 20 tons per acre of 2,000,000 pounds 
of soil. The results are given in table 11 in pH values. 

The lime requirement of the untreated soil determined by a 5-hour Tacke 
run was 3.2 tons per acre. It will be observed that the ammonium sulfate 
alone increased the acidity, as would be expected of a physiologically acid salt 
which has been nitrified. The increased acidity is not overcome by the 1-ton 
treatment of calcium carbonate, but is more than overcome by the 3-ton 
treatment. A neutral reaction is not secured however, until 5 tons are applied 
when it runs beyond neutrality. After 9 tons are applied there is only a small 
increase in alkalinity, and with 20 tons the pH is not quite 8. 

In table 12 similar results are presented from the tests with an organic 
nitrogenous material, albumin, applied at the rate of 1| tons, or approximately 
the equivalent in nitrogen content of the 1 ton of ammonium sulfate. 

The results are very similar to those obtained with ammonium sulfate. 
Accidentally or otherwise, the albumin caused a slightly greater acidity when 
lime was not applied to the soil but in most cases it was less. In other words, 
the same amount of lime permitted less acidity or more alkalinity when 
albumin was used than when an equivalent amount of ammonium sulfate was 
used. This may be due to the fact that not only was nitric acid produced 
from ammonium sulfate but sulfuric acid also remained. When albumin was 
nitrified if any other acid was produced it was in a smaller quantity or more 
slightly ionized than the sulfuric acid from the ammonium sulfate. 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: II 



157 



TABLE 11 
The hydrogen-ion concentration values for the various treatments, incubated 6 weeks 



Lime. 
pH... 





















o 


o 


o 


o 


o 


o 


o 


i 


rf u i 


d u i 


ri u i 


ri u i 


d* 


a* 


nl 


.ti 


k| 


*ffi 


§g 




l£ 


OJ 


°h5 


o 


g 


g 


g 


S 


g 


g 


55 
























1 ton 


3 tons 


5 tons 


7 tons 


9 tons 


12 tons 


5.08 


4.88 


4.98 


5.67 


7.27 


7.86 


7.91 


7.91 



55 



20 tons 
7.96 



TABLE 12 
Hydrogen-ion concentration values with albumin treatments 





SOIL 
ONLY 


SOIL 

ALBU- 
MIN 


son. 

ALBU- 
MIN 


son. 

ALBU- 
MIN 


son, 

ALBU- 
MIN 


son. 

ALBU- 
MIN 


son. 

ALBU- 
MIN 


son, 

ALBU- 
MIN 


son. 

ALBU- 
MIN 


Lime 



5.08 



4.76 


1 ton 
5.27 


3 tons 
6.07 


5 tons 
7.84 


7 tons 
7.96 


9 tons 
7.91 


12 tons 
7.91 


20 tons 
8.02 


pH: 





TABLE 13 
Hydrogen-ion concentration values with various lime applications on soil alone 





son. 


son. 


SOIL 


SOIL 


son. 


son. 


son. 


son. 


Lime 

PH 



4.70 


1 ton 
4.91 


3 tons 
6.55 


5 tons 
7.69 


7 tons 
7.68 


9 tons 
7.90 


12 tons 
8.05 


20 tons 
8.26 



TABLE 14 
Hydrogen-ion concentration values of soils treated with acids and varying amounts of lime 





son. 

ONLY 


ACIDS 
AND 

son. 


ACIDS 
AND 

son. 


Acros 

AND 
SOIL 


ACIDS 
AND 

son. 


ACIDS 
AND 
SOIL 


ACIDS 
AND 
SOIL 


acids 

AND 
SOIL 


acids 

AND 

son. 


Lime 

pH 



4.71 



3.91 


1 ton 
4.31 


3 tons 
6.15 


5 tons 
7.25 


7 tons 
7.55 


9 tons 
7.55 


12 tons 
7.69 


20 tons 
7 79 







TABLE IS 
Hydrogen-ion concentration values on soils variously treated as shown 



AMMONIUM SULPHATE 


ALBUMIN 


Soil 


Soil 


Soil 


Soil 


Soil 


Soil 


Soil 


Soil 


Soil 


(NH4) 2 S04 
only 


H 2 S04 
1 ton 


H2SO4 
3 tons 


H 2 S04 
5 tons 


Citric acid 
10 tons 


H2SO4 

3 tons 


H 2 S04 
5 tons 


Citric acid 
7 tons 


Albumin 
alone 


4.98 


4.21 


3.85 


3.62 


5.22 


4.36 


3.65 


4.69 


4.76 



TABLE 16 
Hydrogen-ion concentration values on soils treated with varying amounts of citric acid 



SOIL ONLY 


SOIL 


son. 


son. 


SOIL 


son. 




Citric acid 3 tons 


Citric acid 5 tons 


Citric acid 7 tons 


Citric acid 9 tons 


Citric acid 10 tons 


PB 
4.71 


pH 
5.02 


pH 
5.33 


PH 
5.40 


pn 
5.14 


pn 
5.33 



158 R. E. STEPHENSON 

In table 13 are found the results obtained where the soil alone is given the 
various lime treatments and the hydrogen ion determined. 

Evidently there must have been some variation in the soil as this sample 
seems to be more acid originally. The 3-ton treatment did not produce 
neutrality while the 5-ton treatment produced alkalinity. Apparently about 
4 tons, or a little more than the indicated Tacke requirement, is necessary to 
give a neutral soil. The 20-ton treatment runs above pH = 8 which is rather 
alkaline for a limestone treatment. 

In the next series acids were added equivalent, respectively, to the nitric 
and sulfuric acids which would result if the ammonium sulfate were completely 
nitrified. 

The acids increase the acidity but the 5-ton treatment of limestone gives 
a somewhat alkaline soil (table 14). The higher treatments do not cause as 
great an alkalinity as where nothing but lime is added to the soil, even when a 
large excess of lime is present. Another series was treated with ammonium 
sulfate and mineral and organic acids. Sulfuric and citric acids were used in 
equivalent amounts. 

It is very evident that the 10 tons of citric acid in conjunction with the 
ammonium sulfate did not increase the true acidity of the soil (table 15). 
In fact, it is much less. Neither did the 7 tons used with the albumin cause 
any increase. But the sulfuric acid evidently caused quite a marked increase 
in every case, the increase being somewhat proportional to the amount applied. 
The 3-ton application of sulfuric acid did not have so great an effect in the 
presence of albumin as with ammonium sulfate, but the 5-ton treatment had 
nearly as great an effect. 

Another series was run in which citric acid was used on the soil alone. 

It is very evident again that the organic acid has not increased the acidity 
of the soil, and the largest application has no more effect than the smaller 
ones (table 16). 

These results are in accord with the contention that organic acids do not 
accumulate in soils under conditions favorable to crop production. It is 
very evident that the organic acid used here has oxidized rapidly enough to 
remove all cause for suspicion that ordinary acid soils might owe this char- 
acteristic' to citric acid produced from the decay of organic matter. The 
results agree also with those of Stemple, who used citric, oxalic and acetic 
acids. It is possible, of course, that more stable and active organic acids than 
citric might be produced, and that there might be conditions when such acids 
would contribute to the causing of an acid soil. 

SOURCE OF ORGANIC AND MINERAL ACIDS 

From whence arises the acidity of ordinary agricultural soils has long been 
a somewhat perplexing problem. It is generally believed at the present time 
that most of the acidity, except perhaps that in peat and muck soils, arises 
from some mineral source. The leaching of bases and the consequent accumu- 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: H 



159 



lation of acid silicates and alumino-silicates is doubtless responsible for a 
considerable portion of acidity. The practice of using certain commercial 
fertilizers, such as ammonium sulfate, has caused an acid condition of some 
soils. Thus the accumulation of sulfuric, hydrochloric, or nitric acids even 
in small amounts could cause a marked increase in the harmful effects of an 
acid soil, because such acids are highly ionized and would therefore give a 
high hydrogen-ion concentration. A small amount of such acids would 
undoubtedly do more injury than larger amounts of either acid silicates or 
organic acids. 

It may be easily demonstrated that soils contain acids of very variable 
strengths, the more active ones reacting at once and the very slowly active 
ones only after a much longer period of contact with limestone and water. 



TABLE 17 
Lime requirement at intervals of 3 hours 



son. 

NUMBER 



Loam (lbs.) 

Increase (lbs.) 

Per cent of total. 

Sandy loam (lbs.) 
Increase (lbs.) — 
Per cent of total. 

Sand (lbs.) 

Increase (lbs.) — 
Per cent of total. 

Miami silt (lbs.) . 

Increase (lbs.) 

Per cent of total. 



5000 



77.0 



3700 



59.7 



800 



72.7 



1800 



72.0 



6 HOURS 



6100 
1100 
17.0 

5100 
1400 
22.6 

1100 
300 
27.3 

2300 
500 
20.0 



9 HOURS 



6500 
500 
6.0 

6200 
1100 
17.7 

1100 



2500 
200 
8.0 



6500 

100 
6200 

100 
1100 

100 
2500 

100 



This may be due to the fact that the acids are very slowly soluble, or it may be 
partly because of hydrolytic actions which take place slowly. The data in 
table 17 showing the varying degrees of activity of soil acids, are taken from 
a previous work. 

These results show that from 60 to 80 per cent of the acidity based upon a 
total 9-hour run, reacted during the first 3 hours, while there yet remained 6 
to 18 per cent to react during the last 3 hours of the run, except for the sand 
which was not very acid. Determinations have been conducted a much longer 
period than this and have been found to react slowly even after several days. 
One muck soil with a lime requirement of 15,200 pounds at the end of 3 hours 
gave a 25,400-pound requirement at the end of a 23-hour period. A soil of 
this type, however, is quite different from the ordinary soil, and doubtless 
the organic acids have a part to play in its reaction. 



160 



R. E. STEPHENSON 



THE LOSS OF BASES BY SOILS 

It is not presumed that soils become acid so long as they contain bases 
equivalent to the acids. But the question may arise, do acids increase in 
quantity or do bases diminish in quantity and thus leave a surplus acidity, 
and if either or both changes take place in what manner do they occur? 

The bases such as sodium, potassium and calcium must be held originally 
in some chemical combination, undoubtedly with a silicate or alumino-silicate, 
to form a salt or acid salt. This gives a salt of a strong base and a weak acid 
and should therefore by hydrolysis give up a free base. That such is true 
has been demonstrated experimentally as shown by the data from Steiger's 
work (1) with various natural silicates (table 18). 

TABLE 18 

Alkalinity of natural silicates 



NAME 


FORMULA 


COMBINED 
ALKALI 


EQUIVALENT 

OPNaO 

IN SOLUTION 






per cent 


per cent 


Pectolite 


Ca 2 (Si0 3 ) 3 NaH 


9.11 


0.57 


Muscovite .... 


Al 3 (Si0 4 ) 3 KH 2 


10.00 


0.32 


Natrolite 


Al 2 (Si0 4 ) 3 Na 2 H 4 


15.79 


0.30 


Lintonite 


Al 6 (Si0 4 ) 6 (CaNa 2 ) 3 7H 2 


5.92 


0.29 


Phyogopite .... 


Al(Si0 4 ) 3 Mg 3 KH 2 


9.32 


0.22 


Laumonite .... 


Al 2 Si0 4 Si 3 8 Ca4H 2 


1.00 


0.18 


Lepidolite 


KHLiAl 3 (Si0 4 ) 3 K 3 Li 3 (AlF 2 )Al(Si 3 8 ) 3 


13.00 


0.18 


Elaeolite 


Al 3 (Si0 4 ) 3 Na 3 


21.17 


0.16 


Henlandite. . . . 


Al 6 (Si 3 8 ) 6 (CaNa 2 ) 3 16H 2 


2.00 


0.13 


Orthoclase .... 


KAlSi 3 8 


16.00 


0.11 


Analcito 


NaAl(Si0 3 ) 2 2H 2 


14.00 


0.10 


Oligoclase 


ALNaSi 3 8 Al 2 CaSi 2 8 


9.18 


0.09 


Albite 


AlNaSi 3 8 


12.10 


0.07 


Wernerite 


. Ca 4 Al 6 Si 6 2 5Na 4 Al3Si90 24 


11.09 


0.07 


Leucite 


KAl(Si0 3 ) 2 


21.39 


0.06 


Stibite 


Al 2 (Si 3 8 ) 2 (CaNa 2 ).6H 2 


1.00 


0.05 


Chabazite 


Al 2 Si0 4 Si30 8 (CaNa 2 ) .6H 2 


7.10 


0.05 



These results were obtained by placing 0.5-gm. samples in 500 cc. of water 
and maintaining at a temperature of 70°C. for a month. It is to be expected 
that in the soil it might go on even more readily, since the base would be 
leached as liberated unless perchance it reacted with some acid or protein 
decomposition product to form an insoluble salt. Why all bases do not leach 
with about equal readiness cannot be stated, but potassium seems about the 
least readily leached and calcium most readily leached. When several hundred 
pounds of limestone may be leached out in a single year it is not strange that 
a soil may become rather acid and unproductive in time for that reason. 

There is, therefore, nothing more logical than that with increased weathering 
there should come increased acidity. As long as base-rich minerals are tightly 



EFFECT OF ORGANIC MATTER ON SOIL REACTION: II 161 

cemented together or enclosed within the interstices of a resistant granite or 
other mineral, they are mechanically protected and saved from waste. But 
they are likewise saved from any useful function in the soil either as direct 
plant-food or as a neutralizing agent. Virgin soils are not only more likely 
to contain many minerals rich in unleached bases but they contain much 
organic matter in the process of decay and therefore in a condition to react 
with, and to prevent the leaching of base. With the exhaustion of the organic 
matter there is the accompanying loss of base and therefore a non-productive 
sour soil. • 

Experimental data show that practically any type of soil may become 
acid. But the acidity of different soil types behaves in a different way, as 
may be shown also experimentally. A sandy soil is likely to become acid 
readily because there is not sufficient organic matter to prevent leaching of 
such bases as may occur naturally or may be applied artificially. 

There would probably be little acidity due to organic acids, because there 
would likely be very little organic matter in such a soil and because conditions 
would probably be very favorable to the oxidation of such organic acids as 
might possibly develop. Mineral acids such as the acid silicates, and the 
stronger sulfuric and hydrochloric acids from the application of certain ferti- 
lizers, would likely cause the injurious soil reaction. On a clay soil more 
acid alumino-silicates would be probable. Loam soils and those of yet higher 
organic content might contain organic acids, or at least organic compounds 
capable of combining with base. Such soils remain productive in spite of 
such acidity as may develop because necessary bases for plant growth have 
been prevented from leaching and because the organic matter itself is an 
important source of the essential plant-food, nitrogen. In the growth of 
legumes, however, it is perhaps not a question of nitrogen content, but more 
likely a question of reaction and a supply of mineral plant-food, including 
not only the bases potassium and calcium, but also phosphoric acid. 

GENERAL DISCUSSION 

There are many factors which influence directly or indirectly the reaction 
of soils. It is not alone a question of the production of acids but a question 
of the capacity of the soil to resist changes in reaction caused by the acids 
produced. 

Buffering may be effected by both mineral and organic compounds. Sili- 
cates of bases would be capable of neutralizing strong acids, which is in fact 
a buffering effect. Some of the alumino-silicates no doubt react with either 
acid or base and therefore function doubly, saving base and reducing the true 
acidity. The amino acids and many more complex products of protein 
degradation react in the same manner. The ionization constants for the amino 
acids as either acids or bases are very low but of about equal strength, making 
them ideal buffers. This explains why organic soils and clayey soils should 
show greater power to resist changes in reaction. 



162 R. E. STEPHENSON 

Grain size is of course an important factor in determining its reaction, 
especially with mineral soils. The smaller the grain size the more difficult 
it is to prevent water-logging, and therefore the more difficult to maintain 
conditions favorable to the oxidation of organic acids or other harmful products. 
Though coarse-grained soils readily become acid it is perhaps usually with a 
somewhat different type of acidity. Rahn (3) has already demonstrated the 
close relationship between grain size, moisture content, and bacterial activity. 
This relationship has its influence also upon reaction changes. 

SUMMARY 

1. Highly organic soils and clays exhibit a high degree of buffering, while 
coarse sands show little of this capacity. 

2. Sulfuric acid, or physiologically acid salts such as ammonium sulfate, 
cause a change toward increased hydrogen-ion concentration in soils. Citric 
acid did not increase the true acidity. 

3. Ammonium sulfate caused a greater increase in acidity than did its 
nitrogen equivalent of albumin. 

4. When nitric and sulfuric acids were added to the soils in amounts equiv- 
alent to the acidity which might be produced from the complete nitrification 
of ammonium sulfate, a greater increase was produced in the hydrogen-ion 
concentration of the soil than where the ammonium sulfate was used. 

5. A large excess of pure lime carbonate (20 tons) brought the pH value 
to only a little more than 8.0, which seems to be about the limit of alkalinity 
produced by limestone. 

REFERENCES 

(1) Clark, F. W. 1900 Contributions to chemistry and mineralogy. U. S. Geol. Survey 

Bui. 167, p. 156. 

(2) Fred, E. B., and Davenport, Audrey 1918 Influence of reaction on nitrogen- 

assimilating bacteria. In Jour. Agr. Res., v. 14, no. 8, p. 317. 

(3) Rahn, Otto 1912 The bacterial activity in soils as a function of grain size and mois- 

ture content. Mich. Agr. Exp. Sta. Tech. Bui. 16. 

(4) Stephenson, R. E. 1919 The effect of organic matter on soil reaction. In Soil Sci., 

v. 6, p. 413-439. 

(5) Stephenson, R. E. 1921 Soil acidity and bacterial activity. In Soil Sci., v. 11, 

p. 133-144. 



